Set your oven to 180°C (fan 160°C, gas mark 4).
In a bowl, mix the flour, a pinch of salt, and cinnamon. Rub the cold, cubed butter into the flour with your fingertips until the mixture resembles breadcrumbs. Stir in the oats and half of the sugar
Place the drained rhubarb in a baking dish. Sprinkle the remaining sugar over the rhubarb and mix gently to coat
Evenly spread the crumble mixture over the rhubarb, pressing down lightly.
Place the dish in the oven and bake for 25-30 minutes or until the crumble is golden and the rhubarb is bubbling around the edges
Make the Custard - While the crumble is baking, warm the milk in a saucepan over low heat until just steaming. In a bowl, whisk the egg yolks, sugar, and cornflour until smooth. Gradually pour the warm milk into the egg mixture, whisking constantly. Return the mixture to the saucepan and cook over low heat, stirring continuously, until it thickens. Stir in the vanilla extract.
Once the crumble is ready, remove it from the oven. Serve warm with the freshly made custard poured over the top.